1. Field of the Invention
The present invention relates to a rotationally balanced hydraulic swash plate plunger unit, such as a swash plate plunger pump or swash plate plunger motor, and to a hydraulic continuously variable transmission incorporating the novel swash plate plunger unit. More particularly, the present invention relates to a rotationally balanced hydraulic swash plate plunger unit of a fixed capacity type in which the angle of a swash plate is fixed, and to a hydraulic continuously variable transmission incorporating the swash plate plunger unit.
2. Description of the Background Art
Various hydraulic continuously variable transmissions have been known and used, many of these known transmissions including a combination of a hydraulic pump and a hydraulic motor.
One example of a commercial hydraulic continuously variable transmission was sold by Honda and marketed under the trademark “Hondamatic” on the 2001 Honda Fourtrax Foreman Rubicon™ TRX500 ATV.
Other examples of hydraulic continuously variable transmissions are disclosed in Japanese Disposed-open Patent publications JP-A 6-2753 and JP-B 7-88884, proposed by the applicant of the present application. Each of the hydraulic continuously variable transmissions disclosed by these patent documents comprises a swash plate plunger pump, a swash plate plunger motor and a hydraulic closing circuit for connecting the outlet port and inlet port of the swash plate plunger pump to the inlet port and outlet port of the swash plate plunger motor, respectively, wherein a pump swash plate member is driven by an engine.
The referenced transmissions also include a pump cylinder and a motor cylinder connected to each other above the output shaft of the engine, in which the rotation of a motor swash plate member is restricted, and the angle of a motor swash plate can be changed.
In this hydraulic continuously variable transmission, a pump cylinder and a motor cylinder are connected to each other back to back, and pump and motor distributor valves (distributor valves) are arranged in this connection portion, to constitute a hydraulic closing circuit. The pump and motor distributor valves supply oil discharged from pump plungers which are reciprocated in the pump cylinder by the revolution of a pump swash plate into a motor cylinder chamber to push motor plungers so that they are moved in an axial direction in slide contact with a motor swash plate to drive the motor cylinder.
In the above hydraulic pump, a pump casing is rotatably supported and driven by input rotation, and a swash plate member is installed in this pump casing in such a manner that it is inclined at a predetermined angle. Since this swash plate member is inclined, the shape of the pump casing is not rotationally symmetrical (that is, cylindrical or circular) but rotationally asymmetrical. Therefore, when the pump casing is turned as it is, rotation becomes unbalanced. Accordingly, to prevent this unbalanced rotation, a counter weight for balancing is installed in the pump casing in the previously known transmission.
However, a weight increase is resulted by installing the counter weight, and steps for producing the counter weight separately and for installing it are required, thereby boosting production costs.
Although the known hydraulic continuously variable transmissions have some utility for their intended purposes, there is still a need to provide an improved hydraulic swash plate plunger unit such as a swash plate plunger pump or swash plate plunger motor. More particularly, there is a need for a swash plate servo device for tilting and rolling a swash plate in a hydraulic swash plate plunger unit such as a swash plate plunger pump or swash plate plunger motor.